In a digital communication system, digital symbols representing information are transmitted between different devices, e.g., between a base station device and a mobile terminal, to exchange information.
At the receiver side, the received signal is processed to obtain a sequence or stream of digital samples and these samples may be represented as complex symbols. For example, the received signal may be filtered, amplified, and mixed down to base band using in-phase and quadrature local oscillators, and after analog-to-digital (A/D) conversion and synchronization processing, a stream of complex received symbols is obtained. Each symbol in the complex symbol stream may be represented as a sum of a real part and an imaginary part.
Newer 3rd generation (3G) cellular communication systems employ wideband code division multiple access (WCDMA) technology. An extension of the WCDMA standard is known as high speed downlink packet access (HSDPA) and has recently been standardized within the 3rd Generation Partnership Project (3GPP) standardization organization. HSDPA introduces known technologies such as higher order modulation and incremental redundancy to the 3GPP Universal Mobile Telecommunications System (UMTS) standard. The higher order modulation introduced in HSDPA is M-QAM, and more particularly 16-state Quadrature Amplitude Modulation (16-QAM).
FIG. 2 shows a schematic complex diagram, wherein the horizontal or x-axis corresponds to the in-phase (I) channel (i.e., real part of the complex symbol) and the vertical axis or y-axis corresponds to the quadrature (Q) channel (or imaginary part of the complex symbols). Generally speaking, 16-QAM is achieved by modulating two four-level pulse amplitude modulated (PAM) signals onto two respective orthogonal carriers (I and Q), providing 42=16 possible symbol representations. Accordingly, a 16-QAM symbol includes phase information based on the respective I or Q orthogonal carrier and amplitude information, whereas Quadrature Phase Shift Keying (QPSK) detection, by comparison, includes only phase information.
In FIG. 2, every black area or dot represents one complex symbol with its phase and amplitude information. The amplitude information corresponds to the radial distance of the dot from the origin of the coordinate system and the phase information corresponds to the angle with respect to the positive x-axis. Due to the fact that 16 symbols can be represented in the 16-QAM system, each symbol corresponds to a 4-bit word comprising four bits S0 to S3. The binary representations of the complex symbols are indicated above the dots in FIG. 2.
Receivers in WCDMA systems rely on a reference signal, such as a time-multiplexed pilot symbol or code-multiplexed pilot channel, to calculate estimates of a radio channel's response. Typically, the channel gain and phase of the Common Pilot Channel (CPICH) is estimated once per slot for this purpose. The 16-QAM data, which is transferred on the High Speed Physical Downlink Shared Channel (HS-PDSCH), requires processing of phase and amplitude information to recover the information in the data. While the phase information can be reused from the CPICH based measurements, the gain offset between the CPICH and HS-PDSCH, which is unknown to the receiver, must be determined separately in order to establish an amplitude reference.
Thus, in order to correctly demodulate the transmitted symbols, the amplitude or, equivalently, scaling factor of the transmitted symbols has to be known. In many systems, the amplitude is not known a-priori on the receiver side and therefore has to be estimated. The accuracy of this estimate has direct impact on the link level performance in terms of frame error rate and/or data throughput.
If no information on the amplitude of the transmitted symbols (e.g., relative to known pilot symbols) is signaled, this amplitude has to be “blindly” estimated by the receiver. E.g., based only on the unknown received data symbols. In a real world transmission system, the received symbols are additionally distorted by noise and/or interference with unknown statistical parameters.
However, conventional methods for blind amplitude estimation require quite complex calculations.
The U.S. Patent Application Publication No. 2004/0096015 A1 discloses estimation of the average value of the in-phase/quadrature component of a received signal as well as signal power by averaging the magnitudes of a sufficiently large number of received signal samples. These values are used by a hard decision unit to estimate the amplitude of the data symbols. However, a specific method of calculation of these amplitude estimates is not given or cited.